1. Field of the Invention
The present invention relates to a screen display element for displaying characters (including numerals and marks) on a display device such as CRT.
2. Description of the Prior Art
FIG. 8 is a block diagram showing a structure of a conventional screen display element. In FIG. 8, numeral 1a represents a CROM (character ROM) storing bit pattern data as a component of a character consisted of n.times.m dots, numeral 2 represents a CRAM (character RAM) having addresses corresponding to each display position on the screen for holding addresses for the CROM 1a as data, numeral 7 represents a line selecting section for selecting data for 1 line of necessary scanning line from data output for 1 character in the CROM 1a, numeral 5 represents a P/S converting section (parallel/serial converting section) for converting data outputted in parallel from the line selecting section 7 into serial signals necessary for screen display, and numeral 6 represents a display control section for controlling the CROM 1a, CRAM 2, line selecting section 7, and the P/S converting section 5 and outputting image signals necessary for screen display.
FIG. 9 is a schematic diagram of the main part of the CROM 1a. The CROM 1a comprises a decoder 91 for decoding address from an address line 90, memory cells 93 arranged in matrix storing bit pattern data, data lines 94 for transmitting data of the memory cells 93 read out by signals from word lines 92, and a sense amplifier 95 for amplifying the data in the data lines 94 to logical level.
The structure of the CROM 1a, in case of a character set having 256 characters in which 1 character is represented by 12 dots in width and 18 dots in length, for example, is (12.times.18=) 216 bits.times.256 words, and 216 bits for 1 character among the 256 characters with 8-bit address are read out. A specific address in the CRAM 2 corresponds to a specific position on the screen, and written specific data corresponds to a specific character, i.e. to a specific address in the CROM 1a. For example, when the screen is consisted of 256 characters calculated by a multiplication of 16 columns in width and 16 rows in length where 1 character is made to be 1 word which holds address (256 words=8 bits) as a 8-bit data for the CROM 1a, the structure of the CRAM 2 is of 8 bits.times.256 words. Among the 8-bit addresses, upper 4 bits are used for indicating 1 row among of 16 rows and lower 4 bits are used for indicating 1 column among 16 columns.
FIG. 10 is a flowchart showing the operational procedure in the conventional example and FIG. 11 is an auxiliary illustration for FIG. 10. At steps 101-106 in FIG. 10, the display control section 6 requests the CRAM 2 for data for a line of a scanning line. Then, the CRAM 2 indicates 1 address of the CROM 1a. Thus, the CROM 1a outputs whole bit data of 1 display character to the P/S converting section 5 through the line selecting section 7. The P/S converting section 5 converts data for 1 line of 1 character, which is sent sequentially, into consecutive data for 1 line.
The operation of the conventional screen display element will now be explained referring to FIGS. 8-11.
The data written in the CRAM 2 corresponds to characters (marks and the like) to be displayed on the screen. When a specific character is to be displayed on a specific position on the screen, data corresponding to the specific character is written in address memory corresponding to the specific position in the CRAM 2. The data written in the CRAM 2 corresponds to a specific address in the CROM 1a. For example, it is assumed that data "0" corresponds to a character "A" and data "1" corresponds to a character "B". A specific data is written in a specific address in the CRAM 2 by means of, for example, a CPU (not shown). For example, data "0" is written in an address "0" of the CRAM 2 and data "1" is written in an address "1". The specific address in the CRAM 2 corresponds to the specific position on the screen. Here, the addresses "0" and "1" correspond to 1st column and 2nd column in 1st row respectively on the screen and "AB" will be displayed in upper left hand side on the screen. The display control section 6 makes the CRAM 2, CROM 1a, line selecting section 7, and the P/S converting section 5 output necessary data at necessary timing for screen display, so as to display them on the screen.
Assuming that a real screen display is being performed by scanning line which proceeds from left to right and from upper part to lower part, the procedure for the screen display will be as follows.
When 1st line on the screen is to be indicated (steps 101, 102, 103), the display control section 6 supplies sequentially addresses corresponding to the 1st row to the CRAM 2. Data which represents a character in the 1st row which is read out sequentially from the CRAM 2 is supplied sequentially to the CROM 1a as an address, and bit pattern data 216 bits of the character to be displayed in the 1st row is read out from the CROM 1a. The display control section 6 selects a first 12 bits necessary for displaying the 1st line from among 216 bits which are sequentially read out by the line selecting section 7, and outputs it. The P/S converting section 5 converts 12-bit parallel data outputted sequentially from the line selecting section 7 into serial data sequentially so as to output it. The display control section 6 converts the serial data outputted from the P/S converting section 5 into image signal necessary for screen display and outputs it to a CRT which is not shown, so as to perform screen display.
When 2nd line on the screen is to be displayed (steps 104, 105, 106), although it is the same as the case of the 1st line, the display control section 6 selects 2nd 12-bit necessary for displaying the 2nd line from among 216 bits read out sequentially by the line selecting section 7, and outputs it.
Accordingly, when an arbitrary Nth line on the screen is to be displayed, the display control section 6 supplies sequentially an address corresponding to {N-1).div.18+1}th row to the CRAM 2. Data representing the character of 1st row read out sequentially from the CRAM 2 is supplied sequentially to the CROM 1a as address, and bit data 216 bits of the character to be displayed in {(N-1).div.18+1)}th row is read out sequentially from the CROM 1a. The display control section 6 makes the line selecting section 7 select sequentially {(N-1).div.18+1)}th 12-bit necessary for displaying Nth line from among 216 bits read out sequentially from the CROM 1a and make it outputted, and the P/S converting section 5 converts parallel data of 12-bit selected and outputted sequentially from the line selecting section 7 into serial data sequentially so as to output. The display control section 6 converts the serial data outputted from the P/S converting section 5 into image signal necessary for displaying on the screen so as to make the CRT (not shown) display the character.
Supposing that character information having such a bit structure shown in FIG. 12 as a combination of 14 sets of 1 row in width, 1st row and 5th-12th rows, and 3rd row and 4th row have the same number of bits, respectively. That is, such a character information has a plurality of data with the same bit structure, so that it provide a high redundancy inevitably. Accordingly, although the conventional screen display element such as described above has the high redundancy of the character information in structure, all characters should have data having the same number of bits. As a result, there is a problem that the capacity of the CROM for storing the data will be necessarily enlarged.